Well, I beg to differ there.
At least all my rabbits are immune to that tonic immobility, I've never seen anything like it. Oh, yes, they are startled for a moment when not used to it, but they are not in any way tranced or something. "Studies show", pardon me, but I got weary of that term. And, as said, they get used to it, so it's just something new, unusual, maybe uncomfortable, but a far cry from being freaked out or immobile due to neurological issues. Taking some laboratory rabbits and putting them on their back, taking their blood for sure will produce stress, although I doubt someone actually did that.
Them being prey animals, no predator puts them on their back. That has nothing to do with it. Predators don't turn chicken on their back either. What is true that rabbits very much dislike not being in control.
That "cruel" is a slapped on human thing, by people with an agenda. My rabbits don't like it, but they don't hold a grudge. And they are not scared while, and after I hold them that way, slightly grumpy when I do it, but that's it.
EDIT: Just to add, I don't carry them around like babys to show off, or play with them that way, that's stupid and really not necessary, but sometimes it is a useful way to hold them.